Q: What does A-Level Physics: 13) Thermodynamic Systems Guide cover? A: Internal energy, pΔV work, the Zeroth & First Laws, plus specific heat and latent heat - this guide unpacks Topic 13 of the 2026 H2 Physics syllabus.
TL;DR Thermodynamics is the energy balance sheet of the universe. Nail the distinctions between U, Q, W and you will unlock marks in gas laws, kinetic theory and every practical calorimetry task.
1 Internal energy (U)
The macroscopic state of a system fixes its internal energy: the sum of random microscopic kinetic (Ek) and potential (Ep)
energies. No single thermometer can reveal
U
directly - you must track
how
energy enters or leaves.
1.1 Parent takeaway
A student who writes “heat stored in the gas” is forfeiting method marks. Heat is a transfer, not a store.
1.2 Mini-drill
State whether each contributes to U:
Scenario
Contributes to \(U\)?
Translational motion of the whole cylinder
No
Vibrations between gas molecules
Yes
2 Thermodynamic temperature (T)
Absolute temperature in kelvin is directly proportional to the mean kinetic energy per particle. Doubling T doubles ⟨Ek⟩. Always quote exam answers to 2-3 s.f. unless otherwise stated.
3 Heating (Q) vs work (W)
Symbol
Process
Positive when...
Typical formula
\(Q\)
Energy transfer by heating
Energy enters system
\(Q = mc \Delta T\)
\(W\)
Mechanical work
Done on system
\(W = p \Delta V\) (constant \(p\))
Sign hack: In SEAB mark schemes, work done by the gas is negative.
4 Zeroth law - the thermometer principle
If A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C. This justifies using a third body (a thermometer) to compare temperatures.
5 First law - the energy ledger
ΔU=Q+W.
Positive Q: heating adds energy.
Positive W: surroundings compress system.
For an ideal gas in free expansion, Q=0 and W=0, hence ΔU=0: temperature unchanged.
6 Specific heat capacity (c)
Defined as the heat required per unit mass per kelvin rise:
Q=mcΔT.
6.1 Quick-check
Water has c≈4.18kJ⋅kg−1⋅K−1 - roughly 30x that of copper. That is why water is a coolant.
7 Specific latent heat (L)
Energy per unit mass for a phase change at constant T:
Q=mL.
Common values:\
Substance
\(L_f\) (fusion) / \(\pu{kJ.kg-1}\)
\(L_v\) (vaporisation) / \(\pu{kJ.kg-1}\)
Water
334
2260
8 WA & Paper 4 timing rules
1 mark ≈ 1.5 min - budget your section time.
Copy units first when using data-book tables.
Show full working; SEAB awards method marks even if arithmetic slips.
Comprehensive revision pack
9478 Section III, Topic 13 Syllabus outcomes at a glance
Outcome (a) - explain internal energy and thermodynamic temperature.
Outcome (b) - apply the first law of thermodynamics to closed systems.
Outcome (c) - calculate heating and work done in simple processes.
Outcome (d) - determine energy changes using specific heat capacity and latent heat.
Outcome (e) - interpret p-V diagrams and identify isothermal, adiabatic, isobaric, and isochoric processes.
Concept map (in words)
Identify the process path (constant T, V, p or adiabatic). Track energy transfers: heating (Q) and work (W) alter internal energy (U). Use c and L for temperature changes and phase changes respectively. p-V diagrams visualise work as area under curve.
Key relations
Quantity/Process
Expression / reminder
First law
\( \Delta U = Q + W \) (\(W\) positive when done on system)
Work at constant pressure
\( W = p \Delta V \) (positive if compression)
Specific heat
\( Q = mc \Delta T \)
Specific latent heat
\( Q = mL \)
Isothermal ideal gas
\( pV = \text{constant} \), \( \Delta U = 0 \)
Isochoric process
\( \Delta V = 0 \Rightarrow W = 0 \)
Adiabatic (ideal gas)
\( Q = 0 \Rightarrow \Delta U = W \)
Internal energy of ideal gas
\( U = \tfrac{3}{2} nRT \) (monatomic)
Derivations & reasoning to master
First law sign convention: derive for compression/expansion, noting SEAB's “work done by gas is negative”.
Work on a p-V diagram: show W=∫pdV and interpret the area under the curve.
Heating vs temperature change: explain why adding latent heat occurs without temperature change.
Adiabatic vs isothermal: compare outcomes for ΔU to emphasise physical differences.
Worked example 1 - heating and work
An ideal gas is compressed slowly from 4.0L to 2.0L at constant pressure 150kPa while 1.6kJ of heat is removed. Find the change in internal energy.
Method: W=pΔV (note sign); apply ΔU=Q+W. Discuss the sign of ΔU (gas cooled).
Worked example 2 - latent heat application
0.45kg of ice at −12∘C is heated to steam at 110∘C. Calculate the total energy required. Use cice, cwater, csteam, Lf, Lv.
Approach: break into stages: warm ice → melt → warm water → vaporise → superheat steam. Sum each Q.
Practical & data tasks
Plot p-V data from a piston experiment using dataloggers; estimate work via trapezium rule.
Carry out a calorimetry experiment to determine specific heat of an unknown metal; include uncertainty analysis.
Observe a pressure cooker in operation; relate pressure increase to boiling point shift using phase diagrams.
Common misconceptions & exam traps
Saying “heat stored in the gas” instead of internal energy increases.
Confusing sign convention for work (especially in expansion scenarios).
Neglecting to convert kPa·L to joules 1kPa⋅L=1J.
Forgetting to include phase-change energy when warming substances across melting or boiling points.
Quick self-check quiz
In an isothermal expansion of an ideal gas, what is ΔU? - Zero.
What does a horizontal line on a heating curve represent? - Phase change at constant temperature (latent heat).
State the first law of thermodynamics. - ΔU=Q+W.
Does compressing a gas adiabatically increase or decrease its temperature? - Increase (internal energy rises).
Which process has zero work: isochoric or isobaric? - Isochoric (volume constant).
Revision workflow
Re-draw schematic p-V diagrams for each thermodynamic process and label Q, W, ΔU signs.
Practise energy accounting problems mixing Q and W until you can do them without hesitation.
Build a table of specific heats and latent heats for common materials (water, ice, steam, metals).
Solve past-paper calorimetry questions and compare marking scheme language to adopt proper phrasing.
Practice Quiz
Test yourself on the key concepts from this guide.
Parents: schedule a 1-hour Thermodynamic Systems clinic two weeks before WA 2.
Students: print the First Law equation and stick it on your water bottle - then apply it to every past-year Qn you meet.
Last updated 14 Jul 2025. Next review: upon release of the 2027 draft syllabus.
